Projections systems have been used for years to project motion pictures and still photographs onto screens for viewing. More recently, presentations using multimedia projection systems have become popular for purposes such as sales demonstrations, business meetings, and classroom sessions. In a common mode of operation, multimedia projection systems receive analog video signals from a personal computer (PC). The video signals represent still, partial-, or full-motion display images of the type rendered by the PC. The analog video signals are converted into digital video signals to control a digitally-driven display object, such as a liquid crystal display (LCD) using thin film transistors (TFT), which form display images.
A popular type of multimedia projection system is a projector that incorporates a light source and optical path components upstream and downstream of the LCD to project the display images onto a display screen. An examples of an LCD projector is sold under the trademark LITEPRO by In Focus Systems, Inc. of Wilsonville, Oreg., the assignee of the present application.
Significant effort has been invested into developing projectors producing high-quality images. However, the optical performance of conventional projectors is often less than satisfactory. For instance, image degradation due to keystone distortion and low image contrast occurs in many conventional projectors.
Keystone distortion occurs when a projector is oriented at an angle to the horizontal during projection. Often such angled orientation causes the width of the projected image field to vary linearly from top to bottom (hence the term "keystone"). Keystone distortion distorts projected images, especially adjacent the top and bottom of the image field, and appears unprofessional. While some conventional projectors have a set keystone correction built into the projector based on a preferred projector mounting angle, such correction does not work when the projector is mounted at an angle other than the preferred. No projector offers convenient, manually operated keystone correction.
Image contrast is also often less than optimal in conventional projectors. Image contrast is defined as the ratio of brightness between the whitest and blackest portions of an image. Degraded contrast often results from polarizing sheets being misaligned with respect to the LCD during projector manufacture. Misaligned polarizers degrade contrast by lightening black areas of an image while darkening white areas. To optimize contrast, some projectors include polarizing sheets that are permanently built into the LCD assembly in an optimal orientation adjacent the LCD. While such LCD assemblies provide improved contrast, they tend to create waste in that the entire LCD assembly is scrapped when one component fails. Thus, in the event of LCD failure, the functional polarizing sheet would also be scrapped.
Thus, in light of these disadvantages, it is a principle object of the present invention to provide a projector with a keystone distortion correction mechanism that can be conveniently adjusted by a projector user.
It is another object of the present invention to provide a projector with a contrast optimization mechanism that can be conveniently adjusted during manufacture or servicing of the projector.
It is a further object of the present invention to provide a projector with a contrast optimization mechanism that permits replacement of individual components of the LCD assembly, without requiring that the entire LCD assembly to be scrapped in the event of component failure.
In accordance with a preferred embodiment of the present invention, a multimedia projector is provided with an optical frame that mounts, from back to front, a light source, a back fresnel lens, a back polarizer, an LCD display object, a front polarizer, a front fresnel lens, and a front lens group for projecting images formed by the LCD display object. To provide keystone distortion correction of projected images, the front fresnel lens is manually and selectively tiltable about an axis extending parallel with the plane of the LCD display object. The selective tilting is accomplished by a manually movable fresnel actuator with a cammed surface. A control arm extends between the fresnel actuator and the fresnel lens, and rides on the cammed surface such that a relatively large movement of the fresnel actuator produces a relatively small adjustment in tilt of the fresnel lens in order to permit fine adjustment of the fresnel lens tilt.
In another aspect of the present invention, manual contrast adjustment is provided by a contrast adjustment mechanism attached to the back polarizer. The adjustment mechanism permits the manual selective tilting of the back polarizer about an axis extending perpendicular to the front polarizer, in order to optimize contrast of images projected by the front lens group.